1. Field of the Invention
This invention relates generally to fasteners and fastening techniques for fastening decking to structural members. More particularly, the present invention relates to techniques for fastening plastic decking to the structural members of a deck.
2. Description of the Related Art
Decks have become an extremely popular addition to the modern home. Typically, the structural members of a modern deck are composed of pressure-treated yellow pine. Pressure-treated wood is preferred for the structure of a deck because of its reasonable cost and high resistance to insect and moisture damage. However, deck surfaces made of pressure treated yellow pine tend to check, split, cup, twist and splinter after prolonged exposure to the elements. In addition, yellow pine is a relatively soft wood and is subject to wear, particularly in high-traffic areas of a deck. In response to the deficiencies of pressure treated yellow pine decking, many homeowners and contractors are now specifying alternative materials for decking, such as redwood, cedar, manmade composite materials or tropical hardwoods.
Manmade composite materials (plastic lumber) are made from plastic such as polyethylene, sometimes in combination with wood particles and/or fiberglass. Frequently, the polyethylene is from recycled products such as plastic bags or the plastic jugs used to store water and milk. The fact that plastic lumber is produced from materials that would otherwise be burned or placed in landfills appeals to many ecologically minded homeowners. Plastic lumber does not absorb water, rot or splinter. It never needs staining or painting, although it accepts and holds paint as well as wood.
Conventional tools and fasteners can be used to install plastic lumber. Plastic lumber is extruded in a wide variety of standard and custom shapes, then cut to a user-specified length. Plastic lumber lacks some of the rigidity and strength of wood and therefore cannot currently be used for constructing the structural portion of a deck. Decking, railings and banisters are popular applications for plastic lumber. Joists used to support plastic decking are frequently required to be closer together than the 16 or 24 inches required for {fraction (5/4)} and 2xc3x97 wood decking, respectively. More joists make for a more expensive installation having more screws joining the decking to the structure. Plastic lumber currently costs significantly more than pressure treated decking, but is priced competitively with redwood or tropical hardwood decking. Because of the increased initial cost, plastic decking is frequently chosen for high-end deck installations.
Currently, the practice is to use conventional decking screws to fasten the plastic decking to the deck structural members. Plastic decking accepts screws well and the plastic material has self-lubricating properties that permit the screw to be easily driven below the surface of the decking. It is also possible to over-drive the screw, where the head of the screw passes most of the way through the decking. In addition, the plastic displaced by the shank of the screw has a tendency to xe2x80x9ccreepxe2x80x9d up the shank of the screw during installation. The head of the screw tends to displace plastic radially during countersinking, often resulting in an irregular raised ring around the sunken head of the screw, which is commonly referred to as a xe2x80x9cvolcanoxe2x80x9d.
To provide the finished appearance expected in a quality installation, the builder must then hammer or punch the material down over each screw. In some cases, the plastic material covers the screw head neatly. In other cases, a rough looking opening is left behind. When using a hammer, the worker must be careful to strike the deck surface flush to prevent leaving a hammer mark. Use of a punch eliminates the hammer mark problem, but either approach requires an extra installation step for each screw. Because decks using plastic decking require more floor joists, there are a greater number of screws to install. Even a modest deck requires hundreds of deck screws, making each additional step very expensive in terms of added labor.
There is a need in the art for a screw for use in conjunction with plastic lumber that can be installed in a single step and leaves a finished appearance without further treatment.
Briefly stated, the invention in a preferred form is a screw incorporating a sharp point angle, undercut head and a boring portion to contain displaced decking material below the screw head. As a result, a single-step installation produces a neatly countersunk appearance.
The screw head includes a circumferential undercut connected to the top surface of the screw head by a periphery oriented parallel to the screw axis. Converging conical surfaces define the v-shaped undercut that connects the periphery of the screw head to the shank of the screw. The junction of the periphery with the radially outer conical surface of the undercut forms a peripheral lip. In an alternative embodiment, this lip may be provided with unidirectional cutting teeth that initially improve countersinking and later help prevent the screw from backing out.
What would typically constitute an unthreaded portion of a deck screw in the prior art (extending from the head to the threaded portion of the shank) is in accordance with the present invention replaced with at least one radially projecting helical flute. The helical flute has a helical orientation opposed to that of the screw thread. The helical flute forms a boring portion that, during screw installation, pushes displaced deck material away from the surface of the decking while opening a larger diameter hole beneath the screw head. Decking material displaced by countersinking the screw head is contained beneath the head by the undercut head configuration. The material contained beneath the screw head during installation at least partially fills the hole reamed by the boring portion. A precisely defined sharp point angle and a sharp thread combine to help prevent material creep up the screw shank.
A single step installation of a screw in accordance with the present invention produces a neatly countersunk flush appearance.
An object of the present invention is to provide a new and improved fastener for fastening plastic or composite decking to a structural member.
Another object of the present invention is to provide a new and improved fastener that presents a finished appearance when installed through plastic or composite decking in a single efficient step.
A still further object of the present invention is to provide a new and improved fastener that does not exhibit the xe2x80x9cvolcanoxe2x80x9d effect when used in conjunction with plastic or composite lumber.